Radiant tube burner systems are well know as generally disclosed in various U.S. Patents assigned to the present assignee, Eclipse Combustion, Inc., including U.S. Pat. No. 4,673,350 to Collier; U.S. Pat. No. 4,705,022 to Collier; U.S. Pat. No. 5,241,949 to Collier; and U.S. Pat. No. 6,024,083 to Smirnov. The entire disclosures of all of these patents are hereby incorporated by reference. As generally disclosed in these patents, a radiant tube burner system generally comprises a burner having a combustion air inlet, a fuel inlet, and a burner head that extends into to a radiant tube (often a U-shaped or W shaped tube). The radiant tube is mounted through a furnace wall with inlet and outlet openings on the outside of the furnace. The advantage of using a radiant tube is that the internal environment of a furnace may be kept as a clean environment substantially free of products of combustion, known as flue gas. During operation, the burner convey combustion oxidant (e.g. combustion air generated by a blower) and gaseous fuel to the burner head for combustion and into the radiant tube, where heat is radiated. The products of combustion or flue gases are then conveyed to an exhaust stack for exhausting outside the factory into the ambient environment.
As with any industrial burner, the products of combustion or flue gases as they are known contain nitrous oxides (NOx) emissions, which are undesirable and regulated by regulatory agencies. As a result there is a desire (and a need in some instances where emissions are too high) to reduce nitrous oxides (NOx) emissions.
A well known method for reducing nitrous oxides (NOx) emissions in industrial burners is to recirculate a portion of the flue gas to reduce interaction between oxygen and gaseous fuel and thereby lower the temperature, which inherently lowers the nitrous oxides (NOx) emissions. Proposals for recirculating a portion of the flue gas in radiant tube burners are disclosed in U.S. Pat. No. 6,190,159 to Moore et al. and U.S. Pat. No. 4,800,866, and the entire disclosures of these patents is hereby incorporated by reference. These proposals appear to have several drawbacks relating to complexity and cost, and might be difficult to implement and control.